feat: Support for PNG Images with Alpha Channel in labelme2coco.py (OSError: cannot write mode RGBA as JPEG)

[AddictionLord]

Description:
Currently, labelme2coco.py fails when processing PNG images with alpha channels, raising the following error: OSError: cannot write mode RGBA as JPEG.

This is because JPEG does not support alpha channels. It would be beneficial if the script could automatically handle images with alpha channels, either by converting them to RGB before saving as JPEG, or by saving such images as PNG.

Proposed Solution:
Add support for images with alpha channels by converting RGBA images to RGB before saving as JPEG

Motivation:
This feature would allow users to seamlessly process datasets containing both JPEG and PNG images, improving the robustness and usability of the conversion script.

Contribution:
I am willing to contribute this feature and already have a working implementation that supports both PNG and JPEG images. Please let me know if you would like me to open a pull request with these changes.